The aspartic protease renin plays a major role in the regulation of blood pressure by producing angiotensin I from angiotensinogen. Many studies have identified endothiapepsin (an aspartic proteinase from the fungi endothia parasitica) as an analogue of human renin, since both bind the same inhibitors with similar affinities. A successful strategy in the design of aspartic proteinase inhibitors has been the attempts to mimic the putative tetrahedral transition state (-C(OH)2-NH-). We have studied the binding of three of these inhibitors to endothiapepsin by means of isothermal titration calorimetry and structure based thermodynamic analysis. In two of the inhibitors the hydrolysable peptide bond (-CO-NH-) has been reduced to -CH2-NH- (H-77 and H-142) while in the third one its has been substituted by a hydroxyethylene isoester (-CHOH-CH2-), H-261. Structurally, the binding process is characterized by the burial of large amounts of apolar surface area, which is in accordance with the large and negative heat capacity change observed experimentally. On the other hand, the binding induces the partial burial of some of the basic residues (mainly histidines) of the inhibitor. As a consequence, it was observed that, at low pH, the binding of the inhibitors is coupled to the release of protons which contribute unfavorably to the overall free energy of binding.